LONDON, March 12 (Reuters) - The radiation leak in Japan
immediately recalls memories of accidents at the Chernobyl and
Three Mile Island power stations, and how it unfolds will be a
critical test for international acceptance of nuclear energy.

The Fukushima incident, brought on by the biggest earthquake
ever recorded in Japan, took a turn for the worse on Saturday
after a blast blew the roof off the facility.

There are direct comparisons with the 1979 disaster at Three
Mile Island in the United States -- in both cases a cooling
fault led to a build up of pressure in the radioactive core and
resulted in a relatively small radiation leak.

Both use water to control the temperature as uranium
degrades in a nuclear chain reaction at the reactors' core,
creating steam which drives a turbine to generate electricity.

The stricken Japanese reactor north of Tokyo has little
parallel, however, with the Soviet plant at Chernobyl, where
fundamental design faults led to a deadly serious of explosions
in 1986, causing hundreds of deaths among emergency workers and
contamination across Ukraine and beyond throughout Europe.

Japan's nuclear agency said the problem at Fukushima rated a
4 on a seven-point scale of gravity, less severe than Three Mile
Island, which was a 5, and well short of Chernobyl, a full 7.

THREE MILE ISLAND

In Japan, the earthquake and possibly the following tsunami
overwhelmed mains and back-up power to the coolant pumps.

At Three Mile Island in Pennsylvania human and technical
error caused a confused response to a similar failure, leading
to over-heating, a meltdown of the nuclear core and a write-off
of the reactor. But, unlike at Chernobyl, there was no breach of
a pressure vessel, the shell which insulates the heart of a
reactor, nor any major radiation leak.

Comparing the problems at Fukushima with Three Mile Island,
Robin Grimes, director of the Centre of Nuclear Engineering at
Imperial College London, said: "There are parallel situations in
terms of some of the processes that have occurred, but not
parallel because in one case it was due to the safety processes
in place and in this case it was natural disaster."
In addition, he said, there was no evidence that the core at
Fukushima had melted, as happened at Three Mile Island.

That would, if it happened, lead to a greater build-up of
radiation within the pressure vessel. Nor was there any sign yet
that inner protective shields were damaged, Grimes said.

"It could be that we've had a breach of a fuel pin, a core
melt. We don't know yet," he said, referring to the uranium fuel
rods which lie at the heart of nuclear power generation.

He said that an increase in radiation in the surrounding
area of eight times the normal, natural level was an indication
that damage to the reactor was so far limited:

"That you're only getting eight times background radiation
outside the exploded structure suggests to me the containment
vessel and the nuclear reactor pressure vessel both remain
intact," he said.

The plants at Fukushima and Three Mile Island used water
cooling technology in common with most nuclear reactors. Their
use of water is slightly different, however.

The Japanese reactor applies heat from the nuclear reactions
in the fuel rods directly to water, creating steam which drives
an electricity-generating turbine. At Three Mile Island, water
under pressure transfers heat to a separate system in a less
direct process for producing steam.

CHERNOBYL

At Chernobyl, in a safety check gone wrong, the operators
deliberately prevented a shutdown of the reactor in a rapidly
escalating fiasco. Runaway nuclear fission reactions created a
build-up of pressure and a massive and deadly radiation leakage
as explosions rippped through a too-feeble reactor shell.

Three Mile Island did not result in a serious leak but was
still damaging at the time for the reputation of nuclear power.

The Fukushima reactor dates to the early 1970s, older than
many still in use, and the industry has since developed "even
safer" reactors better equipped to cool naturally, said Grimes.

Opponents of nuclear power have been swift to use the
problems at Fukushima to reinforce arguments that the hazards of
atomic energy outweigh the benefits. Supporters say that the
ability of Japan's dozens of reactors to survive frequent
earthquakes shows their worth, particularly to countries like
Japan which lack their own reserves of oil, gas and coal.

Paddy Regan, Professor of Nuclear Physics at the University
of Surrey near London, said: "We must remember that there are 55
reactors in Japan and this was a huge earthquake.

"As a test of the resilience and robustness of nuclear
plants, it seems they have withstood the effects very well."